Collision induced dissociation of aluminum cluster ions with chemisorbed oxygen, AlnO+m (n=3–26, m=1,2): Influence of electronic structure on stability

The collision induced dissociation of size selected aluminum cluster ions with chemisorbed oxygen, AlnO+m (n=3–26, m=1,2) by argon has been studied using a low energy ion beam apparatus. The main product from AlnO+ is Al+n−2. Aln−2O+ and Al+n−4 are observed from AlnO+2. These products can be accounted for by loss of Al2O (which is very strongly bound) from the clusters. The experimental results were used to bracket the Aln/O interaction energies at 8.0±1.0 eV for Al+6 and 7.5±1.0 eV for Al+19. These large interaction energies indicate that oxygen is multiply bonded to the clusters. The collision induced dissociation cross sections show a strong cluster size dependence. Minima in the cross sections as a function of cluster size are ascribed to particularly stable clusters. The number of aluminum atoms in the particularly stable AlnO+m cluster increases with the addition of oxygen atoms. The stability of these clusters can be accounted for by the electronic shell model if it is assumed that binding oxygen atoms to the clusters removes electrons from the cluster orbitals so that more aluminum atoms are required to close the electronic shells.